1. Field of the Invention
This invention relates to measuring instruments, and more particularly to improvements in a displacement detecting device of a measuring instrument.
2. Description of the Prior Art
Such measuring instruments have been widely utilized in the industrial field that a measuring element supported in a manner to be movable in directions of three axes perpendicularly intersecting one another is brought into contact with a portion to be measured, and displacement values of the measuring element in the directions of the three axes in this case are detected by displacement detecting devices provided for the directions of the three axes, so as to measure dimensions, shape and the like of the portion to be measured.
In the measuring instrument of the type described, the performance of the displacement detecting device as well as the mechanical construction exert a great influence on the measuring accuracy, with the result that various detecting devices such as an optical type, an electromagnetical type and the like are selectively used with the measuring instruments of various types in accordance with the objects thereof.
Now, when the optical type one is adopted as the displacement detecting device, mainly due to the spatial restrictions, a long main scale is secured to a stationary side of the measuring instrument, a short index scale is mounted to a movable side, and a so-called reflection type optical displacement detecting device is generally adopted in such a manner that the device can move relative to the both scales and is provided thereon with a light emitting element and a light receiving element on the side of the index scale.
In adopting the reflection type displacement detecting device, heretofore, there have been many cases where metallic main scales are used. However, in the case of metallic ones, measurement with very high accuracy has been required from the measuring instruments, and, recently, necessity has been voiced for the large-sized ones, thus presenting the following disadvantages.
Namely, (1) it is very difficult to finish parallel and smooth flat surfaces with high accuracies whereby fringes may be affected. Particularly, the adverse influence is notable when the main scale is long. (2) When the main scale is fixed by the spot welding, distortions may be easily caused to the main scale. The distortions are notable when the main scale is long. (3) When the main scale is mounted to a stone surface plate and the like, the main scale may suffer from a distortion with age because the metal and the stone are greatly different in coefficient of thermal expansion from each other. (4) The fringes are formed by the etching method, whereby side etches become large, which is becoming not negligible from the viewpoint of the accuracy.
Now, these disadvantages have been overcome by making the main scale of glass. However, in this case, the following new problems have arisen.
Namely, in the reflection type one, a value of change in the reflected light obtained by the reflection of the fringes of the main scale is detected by a light receiving element. However, when the main scale is made of glass, the light transmitted through non-reflecting portions of the fringes has been reflected by a substance on the rear surface of the main scale (on the side opposite to the index scale) for example, and this reflected light has been received by the light receiving element. The influence of the reflection of the light transmitted through the above-mentioned non-reflecting portions (which is called a disturbance) is greatly influenced by the conditions of the rear surface of the main scale, namely, the conditions (optical property) of the reflecting surface when the light transmitted through the non-reflecting portion is reflected. As the result, an S/N ratio is fluctuated in various ways, thus giving an adverse influence to the measuring accuracy.
Now, in the coordinate measuring instrument, normally, main scales of displacement detecting devices are mounted such that a main scale for detecting in a direction of the X-axis is mounted on a guide member made of stainless steel, another main scale for detecting in a direction of the Y-axis is mounted on a stone surface plate, and a further main scale for detecting in a direction of the Z-axis is mounted on a spindle made of the other material. In consequence, when main scales made of glass are adopted, even if the displacement detecting devices identical with one another are mounted to one and the same coordinate measuring instruments, the reflected lights are varied in their intensities and modes from one another due to the differences in optical property of the portions to which the main scales are mounted, whereby it becomes necessary for the light receiving elements to be particularly adjusted.
There are some cases where the optical properties of the portions to which the main scales are mounted may change with age. In that case, even if the particular adjustments would be made, lowered accuracies could not be avoided. Moreover, there are many cases where it is unclear what adjusting method should be fitted for the optical property of the portions to which the main scales are mounted. In that case, it is impossible to make the adjustments.